This problem occurs because the mechanic is unaware there is a washer there. When the plug is removed the washer initially sticks to the block. As the coolant gushes out it frees the washer and deposits it in the container the coolant is being collected in, where it is subsequently dumped. Then the "where is the seal" questions arise.

I think a quick call to your nearest Mercedes-Benz dealer should solve the problem. He should also be able to get you a seal for a quarter or two. Good Luck, and I hope this helps, Jim